1. Field of the Invention
The present invention relates to a camera-attachment lens system which is used instead of an eyepiece optical system of a vision-optical system, such as an astronomical telescope or a spotting scope (terrestrial monocular), and is arranged to re-form an image formed by an objective optical system.
2. Description of the Prior Art
In commercially-available attachment lens systems of the above type, the correcting of field curvature and astigmatism are not sufficiently made when these attachment lens systems are actually used, so that the formed image thereby are prone to be blurred at the periphery of the view field. Accordingly, such a poor image quality does not satisfy the users of thereof.
The object of the present invention is to provide a camera-attachment lens system which can reduce field curvature and astigmatism, and can maintain suitable image-forming performance up to the periphery of the view field.
A camera-attachment lens system of the present invention is provided behind an objective optical system, and is arranged to relay a real image formed by an objective optical system, and to re-form a real image. In order to achieve the above object, there is provided a camera-attachment lens system including a positive powered (hereinafter, positive) condenser lens system and a positive image-forming lens system, in this order from the objective optical system; and the camera-attachment lens system satisfies the following conditions:
1.5 less than fa/fco less than 2.4xe2x80x83xe2x80x83(1)
0.08 less than DO/fa less than 0.16xe2x80x83xe2x80x83(2)
1.65 less than Ncoxe2x80x83xe2x80x83(3)
wherein
fa designates the combined focal length of the condenser lens system and the image-forming lens system;
fco designates the focal length of the condenser lens system;
DO designates the distance, along the optical axis, from the image-forming plane of the objective optical system to the most objective-optical-system side surface of the condenser lens system; and
Nco designates the refractive index of the condenser lens system.
More specifically, the image-forming lens system includes a meniscus first lens element having the convex surface facing toward the objective optical system, cemented lens elements constituted by a negative biconcave second lens element and a positive biconvex third lens element, and a positive fourth lens element, in this order from the objective optical system. The image-forming lens system preferably satisfies the following conditions:
1.60 less than Npxe2x80x83xe2x80x83(4)
0.3 less than fa/f1 less than 1.3xe2x80x83xe2x80x83(5)
3.5 less than fa/r1-2 less than 6.5xe2x80x83xe2x80x83(6)
wherein
Np designates the average refractive index of the positive lens elements in the image-forming lens system;
f1 designates the focal length of the first lens element of the image-forming lens system; and
r1-2 designates the radius of curvature of the surface of the first lens element, the surface of which faces toward the re-image-forming plane.
As explained, the camera-attachment lens system of the present invention is an optical system which is used instead of an eyepiece optical system provided behind the objective optical system in the vision-optical system, and is arranged to relay and to re-form an image formed by the objective optical system. More specifically, with respect to a lens arrangement, the camera-attachment lens system includes the positive condenser lens system and the positive image-forming lens system, in this order from the objective optical system. The image-forming lens system includes the meniscus first lens element having the convex surface facing toward the objective optical system, the cemented lens elements constituted by the negative biconcave second lens element and the positive biconvex third lens element, and the positive fourth lens element, in this order from the objective optical system.
According to the above arrangement, the camera-attachment lens system, which can reduce field curvature and astigmatism, and can maintain suitable image-forming performance up to the periphery of the view field, can be obtained.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-2878 (filed on Jan. 10, 2001) which is expressly incorporated herein in its entirety.